Aggression is a complex multifaceted behaviour, and each facet is likely to be based on more than one factor. It is the interaction of these several factors coupled with the immediate social environment that causes a potentially aggressive person to act aggressively. Objectives: The objectives of this study are to estimate the prevalence of aggressive behaviour among high school students, to identify the sociodemographic factors that correlate with aggression, to ascertain different types of aggressive behaviour, and to investigate the relation between aggression and post-traumatic stress disorder. Methods: A sample of 320 students [158 males and 162 females] was selected from ten high schools representing the five governorates in Kuwait. A stratified multi-stage sampling technique was used. The self-administered questionnaire consisted of four parts; Part I sociodemographic data; Part II- a modified version of questionnaire on delinquent and violent behaviour [Symptom Check List90 RJ, Part III- questions related to life-time traumatic events, and Part IV-assessment of the intrusion, avoidance and arousal categories of post-traumatic stress disorder [Post-traumatic check list-Civilian version]. Results: The mean age of male and female students was 16.5 and 16.6 years respectively. The mean total aggression score in students with low father's education level was 7.09 versus 5.66 for high level of father's education [Total aggression score=18]. The students of divorced parents showed a relatively higher total aggression score. The mean total aggression score for students with deceased and alive parents was 7.03 and 5.76 respectively. Male students were more frequently involved in minor aggression. Shoplifting was more common among female students. Violence was reported to be higher in male than in female students [53.8% and 17.9%] respectively. Most traumatic personal events had no significant influence on the total aggression score in both sexes. The highest mean total aggression score was 10.48 among male students with 28-58 total score of post-traumatic stress disorder. Conclusions: Aggression is a major problem among Kuwaiti adolescents of both sexes. There are differences in types of aggression between the two sexes. Aggression is highly associated with post-traumatic stress disorder among male and female students.